Up to the present, fire alarms have been popularized that are devices using sensors disposed on the surfaces of the walls or ceilings of houses and function to sense smoke and provide notification to residents (e.g., see Japanese Unexamined Patent Application Publication No. 2010-39936). Such a type of alarm includes a smoke sensing unit configured to sense smoke, a speaker configured to issue the sound of an alarm, and a circuit board configured such that the smoke sensing unit and the speaker are connected thereto, and is thus referred to as a “household fire alarm.” The smoke sensing unit senses smoke that is introduced via an introductory portion. The speaker is normally disposed on the front side of the fire alarm, that is, toward the inside of a room so that the sound of the alarm can efficiently reach residents. The circuit board, together with the speaker, is disposed on the indoor side of the fire alarm so that the line connecting the circuit board to the speaker does not pass through the above-described introductory portion and does not interrupt the entry of smoke (e.g., see FIG. 1 of Japanese Unexamined Patent Application Publication No. 2010-39936).
In wired connection-type household fire alarms, the wiring passes through spaces above the ceilings, which is not suitable for existing houses. On the other hand, wireless-type fire alarms can be easily installed and can thus be used in not only existing and newly constructed houses but can also be used in aggregate buildings.
It is important to design such fire alarms so that they are unnoticeable in order for a passersby to feel a sense of incompatibility and so that they are compact. A design is contemplated in which an antenna for wireless communication to the outside is contained in a fire alarm such that it is not exposed, thereby achieving a reduction in the size of the fire alarm.
When an antenna for wireless communication is contained in the fire alarm disclosed in Japanese Unexamined Patent Application Publication No. 2010-39936, it is considered preferable to dispose the antenna in a space (a space on the rear side of a circuit board, that is, a space near the surface of a ceiling or the surface of a wall) provided as an introductory portion for introducing smoke. The reason for this is that if the antenna is disposed on the front side of the circuit board, that is, on the inner side of a room, it is necessary to ensure a space for the antenna between the circuit board and the front cover of the fire alarm and it is difficult to reduce the size of the fire alarm.
However, when the antenna is disposed on the rear side of the circuit board, there is concern about the reduction in the gain of the antenna. The same problem is not limited to a fire alarm configured to detect smoke, and is common to a fire alarm configured to detect heat as well as to wireless apparatuses configured to send and receive signals when the degree of freedom of the layout of an antenna is low.